Angular velocity sensors (gyro sensors) such as mechanical type utilizing the precession of a rotary body, optical type utilizing a change in the timing for receiving a laser beam circulating in a housing caused by the turn of the housing, and fluid type which injects a gas for sensing onto a heat ray in a housing and detects a change in the amount of injection caused by the turn of the housing relying upon the temperature of the heat ray are generally known. Recently, demand has been rapidly increasing for angular velocity sensors used in car navigation systems for detecting the vehicle direction. Further, a vibration-type angular velocity sensor which is inexpensive and light in weight in comparison to the above-mentioned sensors is now commonly being used. The vibration-type angular velocity sensor detects a vibration component (hereinafter also called angular velocity vibration component) when an angular velocity acts upon a vibrator which vibrates in a predetermined reference direction based on the Coriolis' force in a detection direction perpendicular to the reference direction, and produces the angular velocity data based on the vibration component.
Conventional systems for controlling the vehicles utilize the angular velocity sensor in, for example, a vehicle stabilizing control system for maintaining the vehicle in a normal state by detecting transverse skidding of the vehicle and providing optimum control of wheel braking and torque, and a four wheel steering control system which controls the steering angle of the rear wheels or the front wheels of the vehicle. In such systems, an abnormal condition of the vehicle such as the transverse skidding of the vehicle is detected by relying upon angular velocity signals from the angular velocity sensor. Therefore, highly reliable angular velocity signals have been demanded.
One example of the above described conventional system is disclosed in JP-A-2001-153659. In this system, when the driving amplitude of the vibrator goes out of a specified range in the vibration-type angular velocity sensor, abnormal condition occurs at the zero point in the angular velocity sensor output and in the sensitivity. By detecting the driving amplitude, therefore, it is judged whether the driving amplitude is lying within the specified range. Further, when a large shock is imparted to the vehicle when, for example, the vehicle hits a curb, a large shock is also imparted to the angular velocity sensor mounted on the vehicle. In this case, the signal is saturated in the processing circuit of the angular velocity sensor due to the shock, often producing a signal different from the correct angular velocity (yaw rate). Therefore, the system determines that an abnormal condition is occurring in the vehicle control system when the signal of the angular velocity sensor exceeds a predetermined level. Concretely speaking, the driving amplitude of the vibrator is detected by a piezoelectric element and is converted into a charge voltage and is, further, rectified to use a DC signal thereof as an amplitude monitoring signal. An abnormal condition can be detected by checking whether the level of the amplitude monitoring signal is lying within the specified range.
Recently, in order to meet the demands for miniaturization, the piezoelectric vibrator has been replaced by a semiconductor capacitance type vibrator which detects the vibration by capitance change. However, the zero point and sensitivity of this angular velocity sensor is affected by the adhesion of even tiny foreign matters. Therefore, the sensor can not meet the satisfied requirements for detecting the abnormal vibration amplitude of the vibrator and abnormal condition thereof due to disturbance.